Question

Although the valence electron pairs in ammonia have a tetrahedral arrangement, the overall geometric structure of the ammonia molecule is not described as being tetrahedral. Explain.

Short answer

Although the valence electron pairs in ammonia (NH3) have a tetrahedral arrangement due to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the overall geometric structure of the ammonia molecule is not described as tetrahedral. This is because the geometry describes the positions of the atoms, not the electron pairs. The three Hydrogen atoms form a pyramid-like structure around the central Nitrogen atom, resulting in a trigonal pyramid geometry. The lone pair on Nitrogen is not represented as an atom in the structure and is not considered in the overall geometry.

Step-by-step solution

Determine the central atom and its valence electron pairs

First, we must identify the central atom in the ammonia molecule (NH3). In this case, the central atom is Nitrogen (N). Nitrogen has 5 valence electrons. When bonded to three Hydrogen (H) atoms, it forms 3 single bonds by sharing 3 electrons with the Hydrogen atoms. Nitrogen has 2 remaining electrons which form a lone pair not participating in bonding.

Understand VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory is a model used to predict the shape of individual molecules based on the repulsion between electron pairs. According to VSEPR theory, the most stable geometric arrangement of electron pairs around a central atom will have the maximum separation between electron pairs, minimizing electrostatic repulsion.

Apply VSEPR Theory to Ammonia

To apply VSEPR theory to ammonia, we must consider the arrangement of the 3 bonding electron pairs (shared with Hydrogen atoms) and the lone pair of electrons on Nitrogen. VSEPR theory predicts that the 4 electron pairs will be arranged tetrahedrally around the Nitrogen atom since this arrangement allows for maximum separation and minimum repulsion.

Determine the Geometrical Structure of the Ammonia Molecule

Although the electron pairs have a tetrahedral arrangement, the overall geometric structure describes the positions of the atoms, not the electron pairs. The three Hydrogen atoms are bonded around the central Nitrogen atom, forming a pyramid-like structure. Since the lone pair is not represented as an atom in the geometric structure, it is not considered when describing the overall geometry. Therefore, the geometric structure of ammonia is described as a trigonal pyramid, not tetrahedral.